Method for manufacturing a semiconductor device

ABSTRACT

A purpose of the invention is to provide a method for leveling a semiconductor layer without increasing the number and the complication of manufacturing processes as well as without deteriorating a crystal characteristic, and a method for leveling a surface of a semiconductor layer to stabilize an interface between the surface of the semiconductor layer and a gate insulating film, in order to achieve a TFT having a good characteristic. In an atmosphere of one kind or a plural kinds of gas selected from hydrogen or inert gas (nitrogen, argon, helium, neon, krypton and xenon), radiation with a laser beam in the first, second and third conditions is carried out in order, wherein the first condition laser beam is radiated for crystallizing a semiconductor film or improving a crystal characteristic; the second condition laser beam is radiated for eliminating an oxide film; and the third condition laser beam is radiated for leveling a surface of the crystallized semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a thin filmtransistor (TFT) formed by using a crystallized semiconductor for asemiconductor layer including, a channel forming region, a source regionand a source region, particularly, a method for manufacturing asemiconductor having a good crystal characteristic by laser beamradiation.

2. Description of the Related Art

An active matrix type of liquid crystal display device capable of highlyfine display has been manufactured so popularly. In the active matrixtype of liquid crystal display device, a TFT is provided in each pixelin a pixel portion as a switching element for driving a liquid crystal.The TFT provided in the pixel portion is switched between on and off tochange an orientation of the liquid crystal in order to carry outdisplay.

Especially in a crystallized semiconductor film having high electricfield effect mobility (typically, a poly-silicon film) of all others, acarrier moves so fast that, when such crystallized semiconductor film isused for a semiconductor layer including the channel forming region, asource region and a source region, it is possible to provide on asubstrate same as the pixel portion a drive circuit capable ofcorresponding to writing of image data even having high resolution, thedrive circuit being required to operate at high speed. Such crystallizedsemiconductor is on the way to practical use.

Demand of a market, however, does nothing but increase for furtherfineness, higher brightness and lower cost of a liquid crystal displaydevice. In order to solve a cost problem, it is necessary to develop atechnology in which a changeover from a quartz plate, which is expensiveper substrate, and thereby, raises a price of a liquid crystal displaydevice as a final product, to a cheap plate (a glass plate, for example)and a changeover from a high temperature process, which needs amanufacturing cost such as electric power, to a low temperature processare possible.

Therefore, a laser beam radiation method or a crystallization methodusing a catalyst element is used as a method for providing a goodelement at a cheap price and obtaining a good semiconductor film.

Radiation of a semiconductor film with a laser beam enables thesemiconductor film to be crystallized or improved in the crystalcharacter. It is because the semiconductor film is fused due to energyof a laser beam and forms innumerable nucleuses so that respectivenucleuses would grow mainly in a direction parallel to a film surface ofthe semiconductor film to form a crystal particle and be solidified.

In growth of such crystal particle after the laser beam radiation,formed on a semiconductor film a convex portion having the height almostequal to the thickness of the semiconductor film due to a collisionbetween adjacent crystal particles.

In the case that a TFT is manufactured by using semiconductor film,which is obtained in the laser beam radiation process under suchcondition that a convex portion is formed on a surface thereof and onwhich the formed convex portion remains, as a semiconductor layerincluding a channel forming region, a source region and a source region,the roughness of the surface of the semiconductor film is reflected in agate insulating film and a gate electrode, which are later formed on thesemiconductor layer, so that it would cause a problem of dispersion ofan element characteristic.

Further, there are problems that leakage easily occurs in an OFFoperation of a TFT (a drain current flowing in the OFF operation of aTFT becomes high) and that electrostatic focusing occurs to raise an OFFcurrent, since the film thickness of a semiconductor layer is thick atthe convex portion. In addition to the above, the roughness of aninterface between the semiconductor layer and the gate insulating filmtraps a carrier (electron) flowing through the channel forming region sothat the carrier would become a fixed electric charge to vary athreshold voltage, which causes decline in reliability.

On the other hand, a semiconductor film having high electric fieldeffect mobility can be obtained in the laser beam radiation processunder such condition that a convex portion is formed as described above.There is, accordingly, an antinomy relation.

SUMMARY OF THE INVENTION

A purpose of the invention is, in order to practically manufacture a TFThaving a good characteristic, to put into practice a method for levelingthe surface of a semiconductor layer without increasing the number andthe complication of manufacturing processes as well as withoutdeteriorating the crystal characteristic, and a method for leveling asurface of a semiconductor layer to stabilize an interface between thesurface and a gate insulating film.

Another purpose of the invention is to put into practice a method formanufacturing a semiconductor device represented by a liquid crystaldisplay device in which a TFT comprising such semiconductor layer isused for a circuit and/or a switching element.

Thus, the invention is a method for manufacturing a semiconductor devicecomprising steps of:

forming a semiconductor film over an insulating surface;

forming an oxide film on a surface of the semiconductor film: and

radiating the semiconductor film with a laser beam in a first condition,a second condition and a third condition in order in an atmosphere ofone kind of gas or a mixed atmosphere of plural kinds of gas, the gasbeing selected from hydrogen and inert gas,

wherein the laser beam in the first condition is a laser beam having, afirst energy density, a first wavelength, and a first pulse width,

wherein the laser beam in the second condition is a laser beam having anenergy density, a wavelength and a pulse width respectively lower thanthose of the laser beam in the first condition, and

wherein the laser beam in the third condition is under a condition thatthe energy density is higher than that of the first condition by 30 to60 mJ/cm².

The invention is also a method for manufacturing a semiconductor deviceincluding steps of:

forming a semiconductor film over an insulating surface;

forming an oxide film on a surface of the semiconductor film; and

radiating the semiconductor film with a laser beam in a first condition,a second condition and a third condition in order in an atmosphere ofone kind of gas or a mixed atmosphere of plural kinds of gas, the gasbeing selected from hydrogen and inert gas,

wherein the laser beam in the first condition has a first energydensity, a first wavelength and a first pulse width, and radiation ofthe laser beam in the first condition forms a crystallized semiconductorfilm to crystallize the semiconductor film,

wherein the laser beam in the second condition has an energy density, awavelength and a pulse width lower than those of the laser beam in thefirst condition, and radiation of the laser beam in the second conditioneliminates the oxide film, and

wherein the laser beam in the third condition is a laser beam whoseenergy density is higher than that of the first condition by 30 to 60mJ/cm², and radiation of the laser beam in the third condition levels asurface of the crystallized semiconductor film.

Further, the invention is a method for manufacturing, a semiconductordevice including steps of:

forming a semiconductor film over an insulating surface;

forming an oxide film on a surface of the above semiconductor film; and

radiating the semiconductor film with a laser beam in a first condition,a second condition and a third condition in order in an atmosphere ofone kind of gas or a mixed atmosphere of plural kinds of gas, the gasbeing selected from hydrogen and inert gas,

wherein the laser beam in the first condition has a first energydensity, a first wavelength and a first pulse width, and radiation ofthe laser beam in the first condition forms a crystallized semiconductorfilm to crystallize the semiconductor film,

wherein the laser beam in the second condition has an energy density, awavelength and a pulse width lower than those of the laser beam in thefirst condition, and radiation of the above laser beam in the secondcondition eliminates the oxide film, and

wherein the laser beam in the third condition is a laser beam whoseenergy density is higher than that of the first condition by 30 to 60mJ/cm², and radiation of the laser beam in the third condition makes adifference between top and bottom points of a surface of thecrystallized semiconductor film 6 nm or less.

The invention is further a method for manufacturing a semiconductordevice including steps of:

forming a semiconductor film over an insulating surface;

forming a crystallized semiconductor film to crystallize thesemiconductor film;

forming an oxide film on a surface of the crystallized semiconductorfilm; and

radiating the crystallized semiconductor film with a laser beam in afirst condition, a second condition and a third condition in order in anatmosphere of one kind of gas or a mixed atmosphere of plural kinds ofgas, the gas being selected from hydrogen and inert gas,

wherein the laser beam in the first condition has a first energydensity, a first wavelength and a first pulse width, and radiation ofthe laser beam in the first condition crystallizes the semiconductorfilm to form the crystallized semiconductor film,

wherein the laser beam in the second condition has an energy density, awavelength and a pulse width lower than those of the laser beam in thefirst condition, and radiation of the laser beam in the second conditioneliminates the oxide film, and

wherein the laser beam in the third condition is a laser beam whoseenergy density is higher than that of the first condition by 30 to 60mJ/cm², and radiation of the laser beam in the third condition levels asurface of the crystallized semiconductor film.

In the invention, the oxide film is formed by contacting the surface ofthe semiconductor film with a solution or gas containing ozone.

The invention is also a method for manufacturing a semiconductor deviceincluding steps of: forming a semiconductor film over an insulatingsurface;

adding a catalyst element to the semiconductor film so as to form acrystallized semiconductor film by heat treatment;

forming an oxide film on a surface of the crystallized semiconductorfilm;

radiating the crystallized semiconductor film with a laser beam; and

carrying out a heating process in order to transport the catalystelement included in the crystallized semiconductor film to a getteringsite,

wherein the step of radiating the crystallized semiconductor film with alaser beam, radiation with a laser beam in a first condition, a secondcondition and a third condition is carried out in order in an atmosphereof one kind of gas or a mixed atmosphere of plural kinds of gas, the gasbeing selected from hydrogen and inert gas,

wherein the laser beam in the first condition has a first energydensity, a first wavelength and a first pulse width, and radiation ofthe laser beam in the first condition improves a crystal characteristicof the crystallized semiconductor film,

wherein the laser beam in the second condition has a energy density, awavelength and a pulse width lower than those of the laser beam in thefirst condition, and radiation of the laser beam in the second conditioneliminates the oxide film, and

wherein the laser beam in the third condition is a laser beam whoseenergy density is higher than that of the first condition by 30 to 60mJ/cm², and radiation of the laser beam in the third condition levels asurface of the crystallized semiconductor film.

In the invention, the inert gas is selected from the group consisting ofnitrogen, argon, helium, neon, krypton and xenon.

Furthermore, in the invention, the catalyst element is one kind of orplural kinds of element selected from the group consisting of Fe, Ni,Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.

Furthermore, in the invention, the heat treatment is a heating processin which heating wire, lamp light source or heated gas is used as a heatsource.

Furthermore, in the invention, the gettering site includes one kind ofor plural kinds of element selected from the group consisting of He, Ne,Ar, Kr and Xe.

Furthermore, in the invention, the gettering site includes one kind ofor plural kinds of element selected from elements belonging to the 15thgroup in a periodic table.

Furthermore, in the invention, the energy density in the first conditionis 300 to 500 ml/cm².

A semiconductor film can be crystallized by radiation with the laserbeam in the first condition in an atmosphere of one kind of gas or amixed atmosphere of plural kinds of gas, the gas being selected fromnitrogen, hydrogen or inert gas. The surface of the crystallizedsemiconductor film obtained by the first condition laser beam radiationhas a convex portion.

Following to the above, an area radiated with the laser beam in thefirst condition is radiated with a laser beam in the second condition ina process room having the same atmosphere. Thus, an oxide layer isformed on a semiconductor film before the first condition laser beamradiation can be eliminated.

Following to the above, an area, which is radiated with the laser beamin the second condition and in which an oxide layer is eliminated, isradiated with a laser beam in the third condition in a process roomhaving the same atmosphere. Thus, the surface of the crystallizedsemiconductor film can be leveled.

According to the invention, the second condition laser beam radiation iscarried out for an area radiated with the first condition laser beam ina process room having a same atmosphere after the first condition laserbeam radiation is carried out for crystallizing a semiconductor film orimproving the crystal characteristic, and thereby, an oxide film can beeliminated. The surface of the crystallized semiconductor film can bealso leveled by, following to the above, the third condition laser beamradiation of an area, which is radiated by the laser beam in the secondcondition and in which an oxide layer is eliminated, in a process roomhaving a same atmosphere. The laser beam radiation of a semiconductorfilm in order from the first condition, the second condition and thethird condition enables a process from crystallization to leveling to beperformed without changing an atmosphere in a process room, which canshorten time for operation.

Moreover, a process substrate is not contaminated since the process fromcrystallization to leveling can be continuously carried out withoutexposure to the air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment mode of the invention:

FIGS. 2A through 2F illustrate an example of an embodiment mode of theinvention;

FIGS. 3A through 3F illustrate an example of an embodiment mode of theinvention;

FIGS. 4A through 4C illustrate an example of a process for manufacturingan active matrix substrate in accordance with the invention;

FIGS. 5A through 5C illustrate an example of a process for manufacturingan active matrix substrate in accordance with the invention;

FIG. 6 illustrates an example of a process for manufacturing an activematrix substrate in accordance with the invention;

FIG. 7 illustrates an example of a circuit structure of an active matrixsubstrate;

FIGS. 8A through 8E illustrate an embodiment of the invention;

FIGS. 9A through 9C illustrate an embodiment of the invention;

FIG. 10 illustrates an example of a laser beam radiation processingapparatus used in the invention;

FIGS. 11A through 11F illustrate an example of an electronic apparatus;

FIGS. 12A through 12D illustrate an example of an electronic apparatus;and

FIGS. 13A through 13C illustrate an example of an electronic apparatus.

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode 1

A method according to the invention for forming a crystallizedsemiconductor film by continuous radiation with a laser beam in threeconditions so as to level a convex portion of the surface thereof willbe now described with reference to FIG. 1.

First, a ground insulating film (not shown) and an amorphoussemiconductor film 11 are formed on a glass substrate 10. Silicon orSi_(1−x)Ge_(x)(0<x<1) can be used as the semiconductor film. A siliconfilm is used in this mode for carrying out the invention. Second, theamorphous semiconductor film is washed by means of ozone water aspretreatment for laser annealing to form an oxide film (not shown) onthe surface of the amorphous semiconductor film.

Then, in a nitrogen atmosphere, the amorphous semiconductor film isradiated continuously with laser beams in three conditions throughoptical systems (18 a to 18 c) provided to satisfy respectiveconditions. The atmosphere may comprise gas selected from inert gas(argon, helium, neon, krypton or xenon) and hydrogen or mixed gasthereof other than nitrogen.

Radiation with a laser beam in the first condition 12 is first carriedout. A gas laser such as an excimer laser and a solid state laser suchas an Nd:YAG laser and a YLF laser can be used as the laser beam in thefirst condition. The energy density is set to be 300 to 500 mJ/cm² whilethe pulse width is set to be 20 to 30 ns. The amorphous semiconductorfilm is crystallized by radiation with such laser beam in the firstcondition to form a crystallized semiconductor film 13. A convex portionis formed on the surface in crystallizing in the case that the laserbeam radiation is carried out when an oxide film exists on the amorphoussemiconductor film or when the amorphous semiconductor film is easilyoxidized. It is known that a laser beam radiation process in which aconvex portion is formed on the surface of the crystallizedsemiconductor film improves a characteristic of the obtainedcrystallized semiconductor film. Thus, a convex portion exists on thesurface of the crystallized semiconductor film 13 after the process ofthe first condition laser beam radiation. The oxide film is still lefton the crystallized semiconductor film 13.

Radiation with a laser beam in the second condition 14 is then carriedout for an area radiated with the laser beam in the first condition. Alaser having a shorter wavelength, lower energy density and smallerpulse width than the laser beam in the first condition, such as a laseroscillating a beam having a wavelength in an ultraviolet area or avacuum ultraviolet area, is used for the laser beam in the secondcondition. An excimer laser having a short wavelength such as an ArFlaser and a KrF laser may be used, for example. A fourth higher harmonicbeam of a YAG laser may also be used. Such radiation with the laser beamin the second condition is carried out to perform abrasion of the oxidefilm on the crystallized semiconductor film obtained by radiation with alaser beam in the first condition in order to expose a crystallizedsemiconductor film 15. The “abrasion” in this specification means, “amaterial is radiated with a high intensity of laser beam, so that asurface layer of the material would be abraded due to energy absorbed bythe surface or the periphery of the surface”.

The area as radiated with the laser beam in the second condition so thatthe abrasion of the oxide film would have been performed is thenradiated with a laser beam in the third condition 16. A gas laser suchas an excimer laser and a solid state laser such as an Nd:YAG laser anda YLF laser can be used as the laser beam in the third condition. Theenergy density is set to be 30 to 60 mJ/cm² larger than that of thefirst condition. A crystallized semiconductor film 17 having an leveledsurface is accordingly formed by radiation with the laser beam in thethird condition under a condition that the oxide film is eliminated fromthe surface.

A purpose of the radiation with the laser beam in the second conditionis to eliminate the oxide film formed on the surface of a semiconductor(silicon) film. Conventionally, in order to eliminate an oxide filmformed on a surface of a silicon film, a wet process has been carriedout such that a semiconductor film in which an oxide film is formed onthe surface thereof is immersed in an inert hydrofluoric acid, forexample. According to the invention, however, it is possible to make acrystallized semiconductor film surface level by that a first conditionlaser beam is irradiated to an area, for the purpose of crystallizationof a semiconductor film or improvement of a crystal characteristic, asecond condition laser beam is irradiated to the area continuously in aprocess room of the same atmosphere as irradiation process of the firstcondition laser beam to perform abrasion of an oxide film, and then, athird condition laser beam is irradiated to the area which abrasion ofthe oxide film is perfected. Thus, the time for operation can beshortened since it is not necessary to change the atmosphere.

Embodiment Mode 2

In this embodiment mode, an example in which the invention is used forfurther improving the crystal characteristic after a crystallizedsemiconductor film is formed by means of an element for acceleratingcrystallization (referred to as catalyst element, hereinafter) will bedescribed, made with reference to FIGS. 2A through 2F.

A ground insulating film 101 made of a silicon nitride oxide film and anamorphous semiconductor film 102 are formed on a glass substrate 100.The film thickness of the ground insulating film is 200 nm and that ofthe amorphous semiconductor film is 200 nm. The films 101 and 102 can beformed continuously without exposed to the air. Continuous forming of afilm can prevent contamination from occurring. The forming of the groundinsulating film can be omitted in the case of using a quartz substrate.

Then, a catalyst element is added to the amorphous semiconductor film102. In this embodiment mode, nickel is used as a catalyst element andan aqueous solution containing nickel (aqueous solution of nickelacetate) is applied to the film by a spin-coating method to form acatalyst element content layer 103. Iron (Fe), cobalt (Co), ruthenium(Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum(Pt), copper (Cu) and gold (Au) can be used as a catalyst elementalthough nickel is used as the catalyst element in this mode. Sputteringand deposition may be used other than spin-coating in adding a catalystelement.

Next, prior to a crystallizing step, a heating process is carried outfor around one hour at 400 to 500° C. to eliminate hydrogen from thefilm, and then, another heating process is carried out for 4 to 12 hoursat 500 to 650° C. to perform a process for crystallizing thesemiconductor film so that a crystallized semiconductor film 104 wouldbe formed.

Following to the above, an oxide film 105 is formed on the surface ofthe crystallized semiconductor film. As a method for forming an oxidefilm, the surface of the crystallized semiconductor film may beprocessed by means of an aqueous solution, in which an ozone contentaqueous solution, sulfuric acid, hydrochloride acid or nitric acid ismixed with a solution of hydrogen peroxide, or may be formed bygenerating ozone by radiation with ultraviolet rays under an oxygenatmosphere to oxidize the surface of a semiconductor having the crystalstructure. Depositing an oxide film by plasma CVD, sputtering, or vacuumdeposition may also form the oxide film.

The process of radiation with a laser beam in three conditions isperformed here in accordance with the embodiment mode 1 so as to improvethe crystal characteristic of a semiconductor film, to eliminate anoxide film 105 and to level the surface of the semiconductor film, andthereby, a crystallized semiconductor film 106 having an leveled surfaceis formed (FIG. 2C to FIG. 2E).

A step for reducing the concentration of a catalyst element remaining inthe crystallized semiconductor film 106 is carried out after thecrystallized semiconductor is leveled. It is assumed that thecrystallized semiconductor film 106 contains a catalyst element at theconcentration of 1×10¹⁹/cm³or more. It is possible to use thecrystallized semiconductor film 106 on which the catalyst elementremains to manufacture a TFT, but in this case, there is a problem thatthe catalyst element is segregated in a defect of a semiconductor layerand an OFF current unexpectedly rises. Accordingly, a heating process iscarried out for the purpose of eliminating the catalyst element from thecrystallized semiconductor film 106 so that the concentration of thecatalyst element would be reduced to 1×10¹⁷/cm³or less, preferably1×10¹⁶/cm³or less.

A process for reducing the concentration of the catalyst elementremaining on the crystallized semiconductor film 106 to 1×10¹⁷/cm³orless, preferably 1×10¹⁶/cm³or less is called gettering. In uttering,either an element belonging to the 15th group in a periodic table(represented by phosphorus) or an inert gas element may be used.

A method for transporting the catalyst element to a gettering site towhich inert gas is added will be now described.

A barrier layer 107 is formed on the surface of the crystallizedsemiconductor film 106 after the leveling process is completed. Thebarrier layer 107 is provided so that the crystallized semiconductorfilm 106 would not be etched in eliminating by etching a gettering site108 provided later on the barrier layer 107.

The thickness of the barrier layer 107 is around 1 to 10 nm, and thebarrier layer 107 may be easily a chemical oxide formed by processingthe crystallized semiconductor film with ozone water. In anotherexample, the chemical oxide can be formed similarly by means of asolution in which sulfuric acid, hydrochloride acid or nitric acid ismixed with a solution of hydrogen peroxide. In another example, thebarrier layer may be formed by carrying out a plasma process in an oxideatmosphere or ultraviolet rays radiation in an oxygen content atmosphereso that ozone would be generated to perform an oxidation process.Further, in another example, the barrier layer can be formed by a thinoxide film, which is formed by heating at 200 to 350° C. in a cleanoven.

Next, the gettering site 108 is formed on the barrier layer 107 bysputtering. The gettering site 108 is formed by means of a semiconductorfilm containing inert gas at the concentration of 1×10²⁰/cm³or more,represented by an amorphous silicon film, which is 25 to 250 nm inthickness. The gettering site 108 has preferably a low density so that aselecting rate of etching to the crystallized semiconductor film 106would be large since the gettering site 108 is eliminated by etchingafter the gettering step is completed.

The gettering site 108 is formed by sputtering under a condition that Aris 50 sccm, film forming power is 3 kW, temperature of a substrate is150° C. and film forming pressure is 0.2 to 1.0 Pa. In accordance withthe above process, the gettering site 108 containing an inert gaselement at the concentration of 1×10¹⁹to 1×10²²/cm³ can be formed. Theinert gas element does not badly influence the crystallizedsemiconductor film 106 since it is inert in a semiconductor film, andtherefore, the gettering can be performed.

A heating process for ensuring completion of gettering is carried outfollowing to the above. The heating process may be performed by a methodfor heating by means of a furnace or an RTA method in which a lamp orheated gas is used as a heat source. In the case of using a furnace, theheating process should be performed in a nitrogen atmosphere at 450 to600° C. for 0.5 to 12 hours. In the case of the RTA method, asemiconductor film should be heated to around 600 to 1000° C. at amoment.

The catalyst element remaining in the crystallized semiconductor film106 is transported to the gettering site 108 in such heating process, sothat the concentration of the catalyst element in the crystallizedsemiconductor film 106 can be reduced to 1×10¹⁷/cm³ or less, preferably1×10¹⁶/cm³or less. The gettering site 108 is not crystallized in theheating process for gettering. It may be because the inert gas elementis not effused and remains in the gettering site even during the heatingprocess.

The getting site 108 is eliminated by etching after the getteringprocess is completed. Dry etching by means of CIF₃in which plasma is notused or wet etching in which an alkaline solution such as a solutioncontaining hydrazine or tetraethyl ammonium hydroxide ((CH₃)₄NOH) isused can be carried out for the above-mentioned etching. In this etchingstep, the barrier layer 107 works as an etching stopper for preventingthe crystallized semiconductor film 106 from being etched. The barrier107 can be eliminated by means of hydrofluoric acid after theelimination of the gettering site 108 by etching is completed.

As described above, leveling the crystallized semiconductor film andperforming gettering of a catalyst element after the leveling process inaccordance with the invention enables the crystallized semiconductorfilm having a good crystal characteristic and having reducedconcentration of the catalyst element to be manufactured with highquality. When a crystallized semiconductor film is leveled according tothe invention, the problem that a convex portion is formed on thesurface of a semiconductor film due to the laser beam radiation processcarried out for improving the crystal characteristic after acrystallization process by means of a catalyst element can be solved, sothat the catalyst element segregated in the convex portion can besufficiently transported to a gettering site.

In the invention, the catalyst element, which is easily segregated in aconvex portion of a semiconductor film, can be easily eliminated sincethe semiconductor film is leveled in the third condition in the laserbeam radiation step.

Embodiment Mode 3

In this embodiment mode, described an example in which the invention isapplied for further improving the crystal characteristic after acrystallized semiconductor film is formed by a crystallization methodusing a catalyst element different from the mode 2, made with referenceto FIGS. 3A through 3F.

A ground insulating film 201 and an amorphous semiconductor film 202 areformed on a glass substrate 200. The ground insulating film 201 and theamorphous semiconductor film 202 can be formed continuously withoutexposed to the air. The forming of the ground insulating film 201 can beomitted in the case of using a quartz substrate.

A mask 203 made of insulating film is then formed on the amorphoussemiconductor film 202. The mask 203 has 1 μm or more of an opening sothat a catalyst element can be added to a selective area of asemiconductor film.

Next, a solution containing a catalyst element, which is 100 ppm inweight conversion, (aqueous solution of nickel acetate containingnickel, in this mode) is applied by a spin-coating method to form acatalyst element contain layer 204. As for a method for adding acatalyst element, deposition and sputtering may be used other thanspin-coating.

An element such as iron (Fe), cobalt (Co), ruthenium (Ru), rhodium (Rh),palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), copper (Cu)and gold (Au) can be used as a catalyst element.

Prior to a crystallizing step, a heating process step is carried out foraround one hour at 400 to 500° C. After hydrogen is eliminated from asemiconductor film in this heating process, another heating process iscarried out for 6 to 16 hours at 500 to 650° C. to crystallize thesemiconductor film in order to form a crystallized semiconductor film205.

Following to the above, an oxide film 206 is formed on the surface ofthe crystallized semiconductor film. As a method for forming an oxidefilm 206, the surface of the crystallized semiconductor film may beprocessed by means of an aqueous solution, in which an ozone contentaqueous solution, sulfuric acid, hydrochloride acid or nitric acid ismixed with a solution of hydrogen peroxide, or may be formed bygenerating ozone by radiation with ultraviolet rays under an oxygenatmosphere to oxidize the surface of a semiconductor having the crystalstructure. Depositing an oxide film by plasma CVD, sputtering, or vacuumdeposition may also form the oxide film.

The process of radiation with a laser beam in three conditions isperformed here in accordance with the embodiment mode 1 so as to improvethe crystal characteristic of a semiconductor film, to eliminate anoxide film 206 and to level the surface of the semiconductor film, andthereby, a crystallized semiconductor film 207 having an leveled surfaceis formed (FIG. 3C to FIG. 3E).

A heating process for reducing the concentration of a catalyst elementremaining in the crystallized semiconductor film 207 (gettering) iscarried out after the leveling step is completed. It is possible ingettering to use either a method using an element belonging to the 15thgroup in a periodic table (represented by phosphorus) or a method inwhich a catalyst element is transported to a gettering site to whichinert gas has been added.

A gettering method using phosphorus will be described in this mode forcarrying out the invention.

A mask 208 is formed in order to add phosphorus to an area selected fora gettering site. Phosphorus is then added to the crystallizedsemiconductor film 207 to form a gettering site 209. Following to this,a heating process is carried out for transporting the catalyst elementto the gettering site 209.

The heating process for transporting the catalyst element to thegettering site 209 may be performed by a heat-processing method using afurnace or an RTA method in which a lamp or heated gas is used as a heatsource. In the case of using a furnace, the heating process should beperformed in a nitrogen atmosphere at 450 to 600° C. for 0.5 to 12hours. In the case of the RTA method, a semiconductor film should beheated to around 600 to 1000° C. at a moment.

The catalyst element remaining in the crystallized semiconductor film207 is transported to the gettering site 208 in such heating process, sothat the concentration of the catalyst element in the crystallizedsemiconductor film 207 can be reduced to 1×10¹⁷/cm³or less, preferably1×10¹⁶/cm³or less.

After the gettering step is completed, the crystallized semiconductorfilm 207 at a region in which concentration of the catalyst element isreduced to 1×10¹⁷/cm³or less is patterned in a desired shape so that theregion is used for a semiconductor layer. The gettering site containinga catalyst element at high concentration can be eliminated in thepatterning step for forming a semiconductor layer.

As described above, leveling the crystallized semiconductor film andperforming gettering of a catalyst element after the leveling process inaccordance with the invention enables the crystallized semiconductorfilm having a good crystal characteristic and having reducedconcentration of the catalyst element to be manufactured with highquality. It is because the catalyst element segregated in the convexportion can be sufficiently transported to a gettering site, when acrystallized semiconductor film is leveled by the laser beam radiationprocess carried out for improving the crystal characteristic after acrystallization process by means of a catalyst element.

Such use of the invention contributes to sufficiently reduce theconcentration of a catalyst element in a crystallized semiconductorfilm, and thereby, a crystallized semiconductor film having a goodcharacteristic can be obtained. Furthermore, using such semiconductorfilm to manufacture a TFT results in a TFT having low OFF current andhigh reliability.

DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1

An embodiment of the invention will be described with reference to FIGS.4A to 6. A method for simultaneously manufacturing a pixel portion and aTFT of a drive circuit provided in the periphery of the pixel portion(an N-channel type of TFT and a P-channel type of TFT) on a samesubstrate will be described here in detail.

A double layer structure is used for a ground insulating film 301provided on a glass substrate in this embodiment. A single layer filmstructure of the ground insulating film or a structure in which two ormore layers of the ground insulating films are piled can be used,however. A first silicon oxynitride film (component ratio is: Si=32%;O=27%; N=24%; and H=17%), which is formed with SiH₄, NH₃, and N₂O asreaction gas, is formed as a first layer 301 a of the ground insulatingfilm 301 by the plasma CVD method so that the film thickness would be 50nm. Then, a second silicon oxynitride film (component ratio is: Si=32%;O=59%; N=7%; and H=2%), which is formed with SiH₄ and N₂O as reactiongas, is formed as a second layer 301 b of the ground insulating film 301by the plasma CVD method so that the film thickness would be 100 nm.

Next, an amorphous silicon film is formed on the ground insulating film301 by the plasma CVD method so that the film thickness would be 50 nm,and a nickel acetate basic solution containing 10 ppm of nickel inweight conversion is applied on by mean of a spinner. It is alsopossible to spread the nickel element all over the surface by sputteringinstead of applying.

A semiconductor film is crystallized in a heating process to form acrystallized semiconductor film following to the above. An electricfurnace or radiation with an intense beam can be used for the heatingprocess. In the case of using an electric furnace, the heating processis performed at 500 to 650° C. for 4 to 24 hours. In this embodiment, aheating process for crystallization (550° C., 4 hours) is carried out toobtain a crystal silicon film after a heat process for dehydrogenation(500° C., 1 hour). The crystallization may be performed by means of alamp-annealing device although it is carried out in a heating processusing a furnace in this embodiment.

Radiation of a laser beam in three conditions is then performedcontinuously in a nitrogen atmosphere. The atmosphere may comprise onekind of or plural kinds of gas selected from inert gas (argon, helium,neon, krypton and xenon) and hydrogen other than nitrogen. A purpose ofthe first condition laser beam radiation is to compensate a defectremained in a crystal particle for improving a crystal characteristic.Any of an excimer laser beam or a second higher harmonic beam, a thirdhigher harmonic beam and a fourth higher harmonic beam of a YAG laserbeam may be used as the laser beam. An XeCl laser (308 nm in wavelength)is used for the first condition so that the laser beam would be radiatedwith the repeated frequency being at 30 Hz, the energy density at 400mJ/cm² and the pulse width at 30 ns, in this embodiment.

The second condition laser beam radiation is then performed for thepurpose of elimination of an oxide film formed on the surface of asemiconductor film before the first condition laser beam radiation. Forthe laser beam in the second condition, a laser having a shorterwavelength, lower energy density and smaller pulse width than those ofthe laser beam in the first condition is used, and preferably,especially a laser having 248 nm or less of wavelength and 5 ns or lessof pulse width. An ArF laser (193 nm in wavelength) is used for thesecond condition so that the laser beam would be radiated with therepeated frequency being at 30 Hz, the energy density at 300 mJ/cm² andthe pulse width at 5 ns, in this embodiment.

The third condition laser beam radiation is then performed for thepurpose of leveling of a convex portion on the surface of asemiconductor film formed in the first condition laser beam radiation.Any of an excimer laser beam or a second higher harmonic beam or a thirdhigher harmonic beam of a YAG laser beam may be used as the laser beam.A beam emitted from an ultraviolet rays lamp may be used instead of theexcimer beam. The energy density of the laser beam in the thirdcondition is preferably 30 to 60 mJ/cm² larger than that of the firstcondition. An XeCl laser (308 nm in wavelength) is used so that thethird condition laser beam would be radiated with the repeated frequencybeing at 30 Hz, the energy density at 430 mJ/cm² and the pulse width at30 ns, in this embodiment. A value of P−V of an unleveled portion on thesurface of the semiconductor film becomes 6 nm or less after theradiation with the laser beam in the third condition.

The surface of a semiconductor film can be leveled as described above,and thereby, a problem of electric field focusing can be solved.Moreover, the OFF current can be reduced since the film thickness isstable.

Next, a mask comprising a resist is formed after forming a thin oxidefilm on the surface of an obtained crystal silicon film (also called apoly-silicon film) by means of ozone water so as to form semiconductorlayers 302 to 305, which are separated after being etched into a desiredshape in an etching process. The mask comprising a resist is eliminatedafter the semiconductor layers are formed.

It may be also possible to add an impurity element, which gives a P typeor an N type, in order to control a threshold of a TFT (Vth) afterforming the semiconductor layers 302 to 305. Elements belonging to the13th group in the periodic table such as boron (B), aluminum (Al) andgallium (Ga) are known as the impurity element giving a P type to asemiconductor, while elements belonging to the 15th group in theperiodic table, typically phosphorus (P) and arsenic (As), are known asthe impurity element giving an N type to a semiconductor.

The oxide film is then eliminated by means of etchant containinghydrofluoric acid simultaneously with washing of the surface of thesilicon film to form an insulating film, which would be a gateinsulating film 306 and whose basis is silicon. In this embodiment, theinsulating film is formed from a silicon oxide nitride film (componentratio is: Si=32%; O=59%; N=7%; and H=2%) by the plasma CVD method so asto be 115 nm in thickness.

The surface of a semiconductor layer is leveled through the process fromcrystallization (or improvement of a crystal characteristic) to levelingaccording to the invention, in which radiation with the laser beam inthree conditions is performed in order. Thus, an interface between thesemiconductor layer and the gate insulating film is stable and good,which leads to improvement of reliability of a TFT.

Next, a first conductive film 307 having 20 to 100 nm of film thickness,a second conductive film 308 having 100 to 400 nm of film thickness anda third conductive film 309 having 20 to 100 nm of film thickness arepiled on a gate insulating film 306, as shown in FIG. 4A. In thisembodiment, a tungsten film having 50 nm of thickness, an alloy film ofaluminum and titanium (Al—Ti), which has 500 nm of film thickness, and atitanium film having 30 nm of film thickness on the gate insulating film306 are piled in order.

Conductive materials forming the first to third conductive films aremade of an element selected from Ta, W, Ti, Mo, AI and Cu, or an alloymaterial or a chemical compound material using, the element as main. Asemiconductor film represented by a polycrystalline silicon film, whichis formed by doping an impurity element such as phosphorus, may be usedfor the first to third conductive films. For example, tungsten nitridemay be used instead of tungsten for the first conductive film, an alloyfilm of aluminum and silicon (Al—Si) may be used for the secondconductive film instead of the alloy film of aluminum and titanium(Al—Ti), and a film of titanium nitride may be used for the thirdconductive film instead of the titanium film. Further, the structure isnot limited to a triple layer and it may be double layer comprising atantalum nitride film and a tungsten film, for example.

Masks 310 to 314 comprising a resist are formed in an optical exposurestep as shown in FIG. 4B, to carry out a first etching process forforming a gate electrode and a wiring. The first etching process iscarried out under first and second etching conditions. An ICP(inductively coupled plasma) etching method can be used for etching. Theetching condition (electric energy applied to a coil type of electrode,electric energy applied to an electrode on the substrate side,temperature of an electrode on the substrate side, etc.) is properlycontrolled by the ICP etching method so that the film can be etched intoa desired shape of taper. It is possible to use, as etching gas,chlorine gas represented by Cl₂, BCl₃, SiCl₄ and CCI₄, fluorine gasrepresented by CF₄, SF₆ and NF₃, or O₂ properly.

The etching gas to be used is not limited, but using BCl₃, Cl₂ issuitable. Respective gas flow rates are set at 65/10/5 sccm and 450 W ofRF (13.56 MHz) power is given with 1.2 Pa of pressure to a coil type ofelectrode to generate plasma, in order to perform etching for 117seconds. 300 W of RF (13.56 MHz) power is also given to the substrateside (a sample stage) so that a practically minus self-bias voltagewould be applied. An Al film and a Ti film are etched under this firstetching condition to form an end of the first conductive layer into ashape of taper.

The etching condition is changed to the second one after the above, inwhich CF₄, Cl₂ and O₂ are used for the etching gas, respective gas flowrates are set at 25/25/10 sccm, and 500 W of RF (13.56 MHz) power isgiven with 1 Pa of pressure to a coil type of electrode to generateplasma, in order to perform etching for around 30 seconds. 20 W of RF(13.56 MHz) power is also given to the substrate side (a sample stage)so that a practically minus self-bias voltage would be applied. The Alfilm, the Ti film and a W film are all etched to the similar degreeunder the second etching condition in which CF₄ and Cl₂ are mixed. It isbetter to increase the etching time at a rate of around 10 to 20% inorder to carry out etching without any residue on the gate insulatingfilm.

In this first etching process, a shape of the mask comprising a resistis made suitable so that ends of the first conductive layer, the secondconductive layer and the third conductive layer would be in a shape oftaper due to an effect of a bias voltage applied to the substrate side.An angle of the taper portion is 15 to 45°. First shape gate electrodes315 to 319 (first electrodes 315 a to 319 a, second electrodes 315 b to319 b, and third electrodes 315 c to 319 c) comprising the first, secondand third conductive layers are formed in the first etching process, asdescribed above. An area of the gate insulating film, which is notcovered with the first shape gate electrodes 315 to 319, is etched byaround 20 to 50 nm, and thereby, becomes thin.

A second etching process is then carried out without eliminating themasks 310 to 314 comprising a resist, as shown in FIG. 4C, BCl₃ and Cl₂are used for the etching gas, respective gas flow rates are set at 20/60sccm, and 600 W of RF (13.56 MHz) power is given with 1.2 Pa of pressureto a coil type of electrode to generate plasma, in order to performetching. 100 W of RF (13.56 MHz) power is given to the substrate side (asample stage). The second and third electrodes of the first shape gateelectrodes are etched under this third etching condition. The aluminumfilm containing a small amount of titanium and the titanium film arethus anisotropy-etched under the above third etching condition so as toform second shape gate electrodes 320 to 324 (first electrodes 320 a to324 a, second electrodes 320 b to 324 b, and third electrodes 320 c to324 c). An area of the gate insulating film, which is not covered withthe second shape gate electrodes 320 to 324, is etched a little, andthereby, becomes thin.

Then, a first doping process is carried out without eliminating a maskcomprising a resist so as to add an impurity element giving an N type toa semiconductor layer (referred to as an N type of impurity element,hereinafter). The doping process can be performed by the ion-dopingmethod or the ion-implantation method. Phosphorus (P) or arsenic (As) istypically used as the N type of impurity element. In this case, thesecond shape gate electrodes 320 to 323 work as a mask for the N type ofimpurity element, and N types of impurity region 325 to 328 having, afirst concentration are formed in self aligning. The N type of impurityelement is added in a concentration range from 1×10¹⁶to 1×10¹⁷/cm³to theN types of impurity region having the first concentration 325 to 328.

The first doping process is carried out without eliminating a maskcomprising a resist in this embodiment. It can be performed, however,after eliminating the mask comprising a resist.

Following to the above, after the mask comprising a resist iseliminated, masks 329 and 330 comprising a resist are formed as shown inFIG. 5A to carry out a second doping process. A mask 329 protects achannel forming region of a semiconductor layer forming one of theP-channel types of TFT in a drive circuit and its peripheral region. Themask 330 protects a channel forming region of a semiconductor layerforming a TFT in a pixel portion and its peripheral region.

In the second doping process, impurity regions are formed on eachsemiconductor layer, using a difference in the film thickness betweenthe second shape gate electrodes 320 to 324 and the gate insulatingfilm. Phosphorus (P) is not added to an region covered by the masks 329and 330, of course. Thus, N types of impurity region having the secondconcentration 335 and 336 and N types of impurity region having thethird concentration 331 to 334 are formed. An N type of impurity elementis added in a concentration range from 1×10²⁰to 1×10²¹/cm³to the N typesof impurity region having the third concentration 331 to 334. The N typeof impurity region having the second concentration is formed so as tohave a lower concentration than that of the N type of impurity regionhaving the third concentration due to a difference in the film thicknessof the gate insulation film, and an N type of impurity element is addedin a concentration range from 1×10¹⁸to 1×10¹⁹/cm³to the N types ofimpurity region having the second concentration.

Then, after the masks 329 and 330 comprising a resist are eliminated,masks 337 and 338 comprising a resist are newly formed so as to carryout a third doping process as shown in FIG. 5B. In this third dopingprocess, P type of impurity regions having the first concentration 341and the second concentration 339 and 340, to which an impurity elementgiving a P type of conductivity to a semiconductor layer forming aP-channel type of TFT (referred to as a P type of impurity element,hereinafter) is added, are formed. The P type of impurity region havingthe first concentration is formed at an region overlapped with thesecond shape gate electrode, and a P type of impurity element is addedin a concentration range from 1×10¹⁸to 1×10²⁰/cm³to the P type ofimpurity region having the first concentration. A P type of impurityelement is added in a concentration range from 1×10²⁰to 1×10²¹/cm³to theP types of impurity region having the second concentration 339 and 340.The P type of impurity region having the second concentration 339 towhich phosphorus (P) is added in the above-mentioned step has the P typeof conductivity since the P type of impurity element is added 1.5 to 3times in concentration as much as the phosphorus.

The P types of impurity region having the second concentration 342 and343 and the P type of impurity region having the first concentration 344are formed into a semiconductor layer forming a holding capacity in apixel portion.

The N type of impurity region or the P type of impurity region is formedin each semiconductor layer in the steps until the above. The secondshape gate electrodes 320 to 322 are used as the gate electrodes ofrespective TFTs. The second shape gate electrode 323 is used as oneelectrode for forming a holding capacity in a pixel portion.Furthermore, the second shape electrode 324 forms a source wiring in apixel portion.

Next, a step for activation-processing the impurity element added toeach semiconductor layer is carried out. The step for activating theimpurity element is performed by one of a rapid thermal annealing method(RTA method) using a lamp light source or heated gas as a heat source, amethod in which radiation with a YAG laser or an excimer laser iscarried out from the back surface, and a heating process method using afurnace, or by a method combining any of the above. In this embodiment,however, it is important to set a heating process condition so that thesecond conductive layer can undergo the above heating process conditionin the activation step since a material based on aluminum is used forthe second conductive layer.

At the same time as the above activation process, nickel used as acatalyst in crystallization is gettered to the N types of impurityregion having the third concentration 331 to 333 in which phosphorus iscontained at a high concentration and to the P types of impurity regionhaving the second concentration 339 and 342, so that the concentrationof nickel in the semiconductor layer, which is mainly used as a channelforming region, would be reduced. As a result, in a TFT having thechannel fowling region, a value of the OFF current decreases and thecrystal characteristic is good, and thereby, high electric field effectmobility can be obtained, so that a good characteristic of the TFT canbe achieved. In the case that a first gettering step is carried out justafter the leveling process of a semiconductor film as in a method shownin the embodiment modes 2 and 3, the gettering by means of phosphorus isthe second step. The second gettering step is not necessary whengettering is sufficiently performed in the first gettering step.

In this embodiment, an example is described such that an insulating filmis formed before the activation mentioned above. The insulating film maybe formed, however, after completing the above activation.

Next, a first layer insulating film 345 comprising a silicon nitridefilm is formed and a heating process (a heating process at 300 to 550°C. for 1 to 12 hours) is carried out so as to perform a step forproducing a hydride of a semiconductor layer. (FIG. 5C) In this step,dangling bond of a semiconductor layer is terminated by means ofhydrogen contained in the first layer insulating film 345. According tothis step, a hydride of a semiconductor layer can be produced regardlessof existence of an insulating film comprising a silicon oxide film (notshown). In this embodiment, however, it is important to set a heatingprocess condition such that the second conductive layer can undergo theabove heating process condition in the step for producing a hydridesince a material based on aluminum is used as the second conductivelayer. A plasma hydride production method (in which hydrogen excited byplasma is used) may be used as another method for producing a hydride.

Then, a second interlayer insulating film 346 comprising an organicinsulating material is formed on the first interlayer insulating film345. An acrylic resin film having 1.6 μm of film thickness is formed inthis embodiment. A contact hole reaching a source wiring 324 and acontact hole reaching each impurity region are formed following to theabove. Plural etching processes are carried out in order in thisembodiment. In this embodiment, the first layer insulating film isetched with an insulating film (not shown) used as an etching stopperafter the second interlayer insulating film is etched with the firstinterlayer insulating film used as an etching stopper, and then, theinsulating film (not shown) is etched.

A wiring and a pixel electrode are formed by means of Al, Ti, Mo and Wafter the above. It is desirable to use a material superior inreflectiveness such as a film based on Al or Ag and a film of piledlayers thereof for a material of the above electrode and pixelelectrode. Wirings 347 to 352 and a pixel electrode 353 are thus formed.

A drive circuit 405 including a P-channel type of TFT 401 and anN-channel type of TFT 402 and a pixel portion 406 including a pixel TFT403 and a storage capacity 404 can be formed on a same substrate asdescribed above (FIG. 6). Such substrate is referred to as an activematrix substrate for convenience in this specification.

The P-channel type of TFT 401 of the drive circuit 405 includes achannel forming region 354, a P type of impurity region in the secondconcentration 341 a part of which overlaps the first electrode 320 a ofa second shape gate electrode 320, and P types of impurity region in thefirst concentration 339 and 340 working as a source region or a sourceregion. The N-channel type of TFT 402 includes a channel forming region355, an N type of impurity region in the second concentration 335 a partof which overlaps the first electrode 321 a of a second shape gateelectrode 321, and an N type of impurity region in the thirdconcentration 332 working as a source region or a source region. SuchN-channel type of TFT and P-channel type of TFT can form a shiftregister circuit, a buffer circuit, level shifter circuit and a latchcircuit. For the purpose of preventing deterioration caused by a hotcarrier effect, a structure of the N-channel type of TFT 402 is suitableespecially for a buffer circuit whose drive voltage is high.

The pixel TFT 403 (an N-channel type of TFT) of the pixel portion 406includes a channel forming region 356, an N type of impurity region inthe first concentration 327 formed outside the first electrode 322 a ofa gate electrode in the second shape 322, and an N type of impurityregion in the third concentration 333 working as a source region or asource region. A P type impurity region in the first concentration 344and P types of impurity regions having the second concentration 342 and343 are formed on a semiconductor layer working as one electrode of thestorage capacity 404. The storage capacity 404 comprises the secondshape conductive layer 323 and the semiconductor layer 305, with aninsulating film (a film same as a gate insulating film) used asdielectric.

In a pixel TFT of the pixel portion 406, reduction in OFF-state currentand in dispersion is significantly achieved compared with theconventional TFT since a semiconductor layer is level by the thirdcondition laser beam radiation.

Further, forming a pixel electrode by means of a transparent conductivefilm enables a transparent type of display device to be formed althoughone more photo-mask is required.

FIG. 7 illustrates a circuit block showing an example of a circuitstructure of an active matrix substrate. In FIG. 7, formed a pixelportion 601, a data signal line drive circuit 602, and a scan signalline drive circuit 606 in which a TFT is incorporated there.

The data signal line drive circuit 602 comprises a shift register 603,latches 604 and 605 and a buffer circuit. A clock signal and a startsignal are inputted to the shift register 603, while a digital datasignal and a latch signal are inputted to the latches. The scan signalline drive circuit 606 also comprises a shift register and a buffercircuit. The number of pixels of the pixel portion 601 is optional. Inthe case of XGA, 1024×768 pixels can be provided.

Such active matrix substrate enables a display device for active matrixdriving to be formed. A pixel electrode is made of an opticallyreflective material in this embodiment. Thus, a reflective type ofdisplay device can be formed when the pixel electrode in this embodimentis applied to a liquid crystal display device. Using the substratedescribed above, a liquid crystal display device and a light emittingdevice in which an organic light emitting element forms a pixel portioncan be formed. Accordingly, it is possible to manufacture an activematrix substrate corresponding to a reflective type of display device.

Embodiment 2

In this embodiment, the invention can also be applied to a step ofmanufacturing a bottom gate type of TFT. The step of manufacturing abottom gate type of TFT will be briefly described with reference toFIGS. 8A through 9C.

An insulating film such as silicon oxide film, silicon nitride film andsilicon oxide nitride film (not shown) is formed on a substrate 50, andthen, a conductive film is formed and patterned into a desired shape soas to form a gate electrode 51. As a conductive film, an elementselected from Ta, Ti, W, Mo, Cr and Al, or a conductive film based onany of the above elements may be used (FIG. 8 a).

A gate insulating film 52 (52 a, 52 b) is then formed. The gateinsulating film may be in a structure comprising a single layer or piledlayers of a silicon oxide film, a silicon nitride film, or a siliconoxynitride film (FIG. 8 b).

Next, an amorphous silicon film 53 is formed as an amorphoussemiconductor film by a heat CVD method, a plasma CVD method, a pressurereduction CVD method, a vapor deposition method, or a sputtering methodso as to be 10 to 150 nm in thickness. The gate insulating film 52 andthe amorphous silicon film 53 may be continuously formed since it ispossible to form the both by a same forming method. Continuous formingcan prevent the both films from being exposed to the air, and thereby,the surfaces thereof can be prevented from being contaminated, which canreduce characteristic dispersion of a TFT to be manufactured as well asvariation in a threshold voltage.

The amorphous silicon film 53 is then crystallized to form a crystalsilicon film. Any of a method using laser beam radiation, a method usingheat and a method using a catalyst element described in the embodimentmodes 2 and 3 can be selected for the crystallization.

A crystallization method using the laser beam radiation will bedescribed in this embodiment. First, an amorphous semiconductor film iswashed by means of ozone water in a pretreatment of the laser beamradiation so that an oxide film 54 would be formed on the amorphoussemiconductor film. Second, the amorphous semiconductor film is radiatedwith a laser beam in the first condition (an XeCl laser beam (308 nm ofwavelength) at 400 mJ/cm²in energy density and 30 ns in pulse width inthis embodiment) in a nitrogen atmosphere to form a crystallizedsemiconductor film. The surface of this crystallized semiconductor filmformed by the first condition laser beam radiation has a convex portionin which a difference between the top and bottom points thereof isseveral nm to several tens nm (FIG. 8C).

Following to the above, radiation with the laser beam in the secondcondition (an ArF laser beam (193 nm of wavelength) at 300 mJ/cm²inenergy density and 5 ns in pulse width in this embodiment) is carriedout to perform abrasion of the oxide film on the crystallizedsemiconductor film. The oxide film on the crystallized semiconductorfilm is thereby eliminated.

The laser beam in the third condition (an XeCl laser beam (308 nm ofwavelength) at 430 ml/cm²in energy density and 30 ns in pulse width inthis embodiment) is then carried out to level the surface of thecrystallized semiconductor film. A crystallized semiconductor film 56 soobtained and having the leveled surface is used for a semiconductorlayer containing a channel forming region, a source region, and a sourceregion (FIG. 8 d).

Next, an insulating film 57 for protecting a crystal silicon film (achannel forming region) in a later-mentioned step of adding impurity isformed to be 100 to 400 nm in thickness. This insulating film is formedin order to prevent the crystal silicon film from being directly exposedto plasma in adding an impurity element and further in order to enablethe concentration to be fine controlled.

A mask comprising a resist (not shown) is then used to add an impurityelement giving the N type to a crystal silicon film, which is to belater an active layer of an N-channel type of TFT, and an impurityelement giving the P type to a crystal silicon film, which is to belater an active layer of a P-channel type of so as to form a sourceregion 58 a, a source region and 58 c, and an LDD region 58 b.

A step of activating the impurity element added to the crystal siliconfilm is then carried out. Following to this, an insulating film 57 onthe crystal silicon film is eliminated and the crystal silicon film ispatterned into a desired shape before a interlayer insulating film 59 isformed. The interlayer insulating film is formed by means of aninsulating film such as a silicon oxide film, a silicon nitride film anda silicon oxynitride film so as to be 500 to 1500 nm in thickness. Then,a contact hole reaching a source region or a source region of each TFTis formed so that a wiring 60 for electrically connecting respectiveTFTs would be formed.

The invention can be applied regardless of the shape of a TFT, asdescribed above.

Embodiment 3

In this embodiment, an example of an apparatus for carrying out a laserbeam radiation process applicable to the invention will be describedwith reference to FIG. 10.

A laser beam radiation processing apparatus comprises a laser forradiation of a first condition laser beam 701, a laser for radiation ofa second condition laser beam 702, a laser for radiation of a thirdcondition laser beam 703, optical systems 704, 705 and 706 foroscillating laser beams in the respective conditions, and a controllerdevice 707.

A gas laser such as an excimer laser oscillating a light having 400 nmor less of wavelength, and a solid state laser such as an Nd:YAG laserand a YLF laser can be used as the laser beam in the first condition.

A laser having a shorter wavelength, lower energy density and smallerpulse width than the laser beam in the first condition, such as a laseroscillating a beam having a wavelength in an ultraviolet area or avacuum ultraviolet area, is used for the laser beam in the secondcondition. An excimer laser having a short wavelength such as an ArFlaser and a KrF laser may be used, for example. A fourth higher harmonicbeam of a YAG laser may also be used.

A gas laser such as an excimer laser oscillating a light having 400 nmor less of wavelength, and a solid state laser such as an Nd:YAG laserand a YLF laser can be used as the laser beam in the third condition.The energy density is set to be 30 to 60 mJ/cm² larger then that of thefirst condition.

The laser beams in the first and third condition can have a samewavelength. Thus, the laser beams in the first and third conditions maybe laser beams having energy density in the first and third conditions,the beams being separated through an optical system after emitted from asame laser source.

In the case that a laser beam having a same wavelength is used for laserbeams in the first and third conditions, it is possible to first carryout the first and second condition laser beam radiation to provide anoptical system capable of emitting a laser beam in the third conditionto an optical system for emitting a laser beam in the first condition,and then, carry out the third condition laser beam radiation. A personwho carries out the invention can properly determine any condition forthe third condition laser beam radiation other than the order of theradiation.

Optical systems 704 to 706 are provided for focusing and extending alaser beam emitted from a laser so that a surface to be radiated wouldbe radiated with a laser beam having a thin and linear cross-section.FIG. 10 illustrates an example of an optical system comprising acylindrical lens array 708, a cylindrical lens 709, a mirror 710 and atablet cylindrical lens 711. It is possible to carry out radiation witha linear laser beam.

which is around 100 to 400 mm in a longitudinal direction and around 100to 500 μm in a latitudinal direction although this size of the laserbeam depends on the size of lenses. A person who carries out theinvention can use any optical system.

There are other devices provided to the above laser beam radiationprocessing apparatus, such as a nozzle for jetting gas for eliminatingdust on a device substrate 712, a means for supplying the above gas 713,a stage 714, a cassette for storing substrates 718, a means for holdingthe cassette 719 and a means for transportation (scanning) 717 for thepurpose of holding processed substrates and carrying out the radiationin the three laser beam conditions.

The nozzle for jetting gas for eliminating dust on a device substrate712 is also used for the purpose of eliminating a film spattered inabrasion of an oxide film carried out by means of the second conditionlaser beam radiation.

The stage 714 is connected with gas supplying means for supplyingcompressed nitrogen for supplying an atmosphere 715 and 716, so that thegas would be jetted from a small hole provided on a surface of the stage714 to enable a processed substrate to be held in a floating conditionwithout contacting to the stage. Such jet of the gas from the small holeenables an easily bending substrate to be held evenly.

Moreover, holding a substrate in a floating condition can prevent thesubstrate from being contaminated, so that a change in temperature ofthe substrate can be made small, which results in high effectiveness.

A substrate can be taken out from the holding cassette 718 in order tocarry out a laser beam radiation process by means of a transportationmeans 717 provided with an arm. The laser beam radiation process can beperformed all over the process substrate by holding an end portion ofthe substrate by the arm and scanning the substrate in one direction. Acontroller device 707 controls an engaging operation of oscillation of alaser beam and the transportation means.

In the laser beam radiation in three conditions, one substrate may bescanned either once or plural times.

In the case of processing a large substrate having a side longer thanthe longitudinal length of a liner laser beam (for example, a substratein which one side is more than 1000 mm in length and 1 mm or less inthickness), a processing apparatus should be provided with atransportation means, which can transport the substrate in a directionrectangular to one axis direction. In an apparatus including twotransportation means capable of scanning a process substrate in adirection rectangular each other, the laser beam radiation process canbe performed all over the surface of a glass substrate, even when theglass substrate is 1200 mm×1600 mm or 2000 mm×2500 mm in length and 0.4to 0.7 mm in thickness, for example.

The larger the area and the thinner the thickness of a glass substrateis, the easier the substrate bends. However, in a stage for holding asubstrate by means of gas as shown in a processing apparatus used inthis embodiment, the laser beam radiation process can be performed whilean level surface can be kept.

An apparatus shown in this embodiment can be used in embodiment modes 1to 3 and the embodiments 1 and 2.

Embodiment 4

The CMOS circuit and the pixel portion formed by implementing thepresent invention can be used in active matrix type liquid crystaldisplay device (liquid crystal display device). That is, the presentinvention can be applied to all of electronic apparatuses integratedwith such liquid crystal display device at display portions thereof. Byusing the liquid crystal display device formed by using the presentinvention, high definition display can be realized, and further, thedriver circuit can be formed on the same substrate as the pixel portion.Thus, the bigger display portion can be formed.

As such electronic apparatus, there are pointed out a video camera, adigital camera, a projector (rear type or front type), a head mountdisplay (goggle type display), a personal computer, a portableinformation terminal (mobile computer, mobile telephone or electronicbook) and the like. Examples of these are shown in FIGS. 11A through11F, FIGS. 12A through 12D and FIGS. 13A through 13C.

FIG. 11A shows a personal computer including a main body 2001, an imageinput portion 2002, a display portion 2003 and a keyboard 2004. Theliquid crystal display device formed by using the present invention canbe adapted to the display portion 2003.

FIG. 11B shows a video camera including a main body 2101, a displayportion 2102, a voice input portion 2103, operation switches 2104, abattery 2105 and an image receiving portion 2106. The liquid crystaldisplay device formed by using the present invention can be adapted tothe display portion 2102.

FIG. 11C shows a mobile computer including a main body 2201, a cameraportion 2202, an image receiving portion 2203, an operation switch 2204and a display portion 2205. The liquid crystal display device formed byusing the present invention can be adapted to the display portion 2205.

FIG. 11D shows a goggle type display including a main body 2301, adisplay portion 2302 and an arm portion 2303. The liquid crystal displaydevice formed by using the present invention can be adapted to thedisplay portion 2302.

FIG. 11E shows a player using a record medium recorded with programs(hereinafter, referred to as record medium) including a main body 2401,a display portion 2402, a speaker portion 2403, a record medium 2404 andan operation switch 2405. The player uses DVD (Digital Versatile Disc)or CD as the record medium and can enjoy music, enjoy movie and carryout game or Internet. The liquid crystal display device formed by usingthe present invention can be adapted to the display portion 2402.

FIG. 11F shows a digital camera including a main body 2501, a displayportion 2502, an eye contact portion 2503, operation switches 2504 andan image receiving portion (not illustrated). The liquid crystal displaydevice formed by using the present invention can be adapted to thedisplay portion 2502.

FIG. 12A shows a front type projector including a projection apparatus2601 and a screen 2602.

FIG. 12B shows a rear type projector including a main body 2701, aprojection apparatus 2702, a mirror 2703 and a screen 2704.

Further, FIG. 12C is a view showing an example of a structure of theprojection apparatus 2601 and 2702 in FIG. 12A and FIG. 12B,respectively. The projection apparatus 2601 or 2702 is constituted by alight source optical system 2801, mirrors 2802, and 2804 through 2806, adichroic mirror 2803, a prism 2807, a liquid crystal display apparatus2808, a phase difference plate 2809 and a projection optical system2810. The projection optical system 2810 is constituted by an opticalsystem including a projection lens. Although the embodiment shows anexample of three plates type, the embodiment is not particularly limitedthereto but may be of, for example, a single plate type. Further, aperson of executing the embodiment may pertinently provide an opticalsystem such as an optical lens, a film having a polarization function, afilm for adjusting a phase difference or an IR film in an optical pathshown by arrow marks in FIG. 12C.

Further, FIG. 12D is a view showing an example of a structure of thelight source optical system 2801 in FIG. 12C. According to thisembodiment, the light source optical system 2801 is constituted by areflector 2811, a light source 2812, lens arrays 2813 and 2814, apolarization conversion element 2815 and a focusing lens 2816. Further,the light source optical system shown in FIG. 12D is only an example andthe embodiment is not particularly limited thereto. For example, aperson of executing the embodiment may pertinently provide an opticalsystem such as an optical lens, a film having a polarization function, afilm for adjusting a phase difference or an IR film in the light sourceoptical system.

However, according to the projectors shown in FIGS. 12A, 12B and 12C,there is shown a case of using a transmission type electro-opticalapparatus and an example of applying a reflection type electro-opticalapparatus is not illustrated.

FIG. 13A shows a mobile telephone including a display panel 3001, anoperation panel 3002. The display panel 3001 and the operation panel3002 are connected to each other in the connecting portion 3003. In theconnecting portion 3003, the angle θ between a face, which is providedthe display portion 3004 of the display panel 3001, and a face, which isprovided the operation key 3006 of the operation panel 3002, can bechanged arbitrary. Further, a voice output portion 3005, an operationkey 3006, a power source switch 3007 and a sound input portion 3008 arealso included. The liquid crystal display device formed by using thepresent invention can be adapted to the display portion 3004.

FIG. 13B shows a portable book (electronic book) including a main body3101, display portions 3102 and 3103, a record medium 3104, an operationswitch 3105 and an antenna 3106. The liquid crystal display deviceformed by using the present invention can be adapted to the displayportion 3102.

FIG. 13C shows a display including a main body 3201, a support base 3202and a display portion 3203. The display according to the invention isadvantageous particularly in the case of large screen formation and isadvantageous in the display having a diagonal length of 10 inches ormore (particularly, 30 inches or more). The liquid crystal displaydevice formed by using the present invention can be adapted to thedisplay portion 3203.

As has been described, the range of applying the invention is extremelywide and is applicable to electronic apparatus of all the fields.Further, the electronic apparatus of this embodiment can be realized byusing any constitution comprising any combinations of Embodiment modes 1to 3 and Embodiments 1 to 3.

As described above, leveling a semiconductor layer in accordance withthe invention can solve the problem relating to dispersion of an elementcharacteristic caused by surface roughness of a semiconductor film (suchas a problem that leakage easily occurs in an OFF operation of a TFT dueto partially large film thickness of a semiconductor layer, and aproblem that electrostatic focusing occurs to raise an OFF current).

Furthermore, carriers to be trapped decrease as well as variation inthreshold voltage can be held down due to a good interface between asemiconductor layer and a gate insulating film, so that reliability canbe improved.

Using a semiconductor film formed by applying the invention tomanufacture a TFT enables the TFT to have a low OFF current and highreliability.

1. A method for manufacturing a semiconductor device comprising: forminga crystalline semiconductor film over an insulating surface; irradiatingthe crystalline semiconductor film with a laser beam in a gas selectedfrom at least one of a hydrogen and an inert gas to level a surface ofthe crystalline semiconductor film.
 2. The method for manufacturing thesemiconductor device according to claim 1, wherein the inert gas isselected from the group consisting of nitrogen, argon, helium, neon,krypton and xenon.
 3. The method for manufacturing the semiconductordevice according to claim 1, wherein the crystalline semiconductor filmis a silicon film or a Si_(1−x)Ge_(x)(0<x<1) film.
 4. The method formanufacturing the semiconductor device according to claim 1, wherein theinsulating layer is a single layer film structure or a stacked structureof two insulating films formed on a glass substrate.
 5. The method formanufacturing the semiconductor device according to claim 4, wherein athickness of the glass substrate is 0.4 to 0.7 mm.
 6. The method formanufacturing the semiconductor device according to claim 4, wherein theglass substrate is 1200×1600 mm or 2000×2500 mm in length.
 7. The methodfor manufacturing the semiconductor device according to claim 1, whereinthe laser beam is a linear laser beam.
 8. The method for manufacturingthe semiconductor device according to claim 1, wherein the crystallinesemiconductor film is scanned by the laser beam plural times.
 9. Themethod for manufacturing the semiconductor device according to claim 1,wherein the energy density of the laser beam is larger than 300 to 500mJ/cm².
 10. The method for manufacturing the semiconductor deviceaccording to claim 1, wherein the laser beam is one of an excimer laserbeam and a YAG laser beam.
 11. The method for manufacturing thesemiconductor device according to claim 1, wherein the laser beam is aXeCl laser beam.
 12. The method for manufacturing the semiconductordevice according to claim 1, wherein the crystalline semiconductor isheated at 450° to 600° after leveling the surface of the crystallinesemiconductor.
 13. The method for manufacturing the semiconductor deviceaccording to claim 1, wherein the crystalline semiconductor is heated bya RTA method after leveling the surface of the crystallinesemiconductor.
 14. The method for manufacturing the semiconductor deviceaccording to claim 1, after irradiating the crystalline semiconductorfilm, a difference between top and bottom points of the surface of thecrystalline semiconductor film is 6 nm or less.
 15. The method formanufacturing the semiconductor device according to claim 1, wherein anenergy density of the laser beam is 430 mJ/cm² and a pulse width of thelaser beam is 30 ns.
 16. The method for manufacturing the semiconductordevice according to claim 1, further comprising a step of hydrogenatingthe crystalline semiconductor film.
 17. The method for manufacturing thesemiconductor device according to claim 1, wherein the gas is jetted tothe crystalline semiconductor film from a nozzle.
 18. The method formanufacturing the semiconductor device according to claim 1, wherein agas is jetted to the crystalline semiconductor film from below thecrystalline semiconductor film.
 19. The method for manufacturing thesemiconductor device according to claim 1, wherein the semiconductordevice is a display device.
 20. The method for manufacturing thesemiconductor device according to claim 1, wherein the semiconductordevice is an active matrix type liquid crystal display device.
 21. Themethod for manufacturing the semiconductor device according to claim 1,wherein the semiconductor device is an electronic apparatus selectedfrom the group of a video camera, a digital camera, a rear type or fronttype projector, a head mount display, a personal computer, a portableinformation terminal, a mobile computer, a mobile telephone and anelectronic book.
 22. A method for manufacturing a semiconductor devicecomprising: forming a crystalline semiconductor film over an insulatingsurface, wherein a surface of the crystalline semiconductor film has aroughness; irradiating the crystalline semiconductor film with a laserbeam in a gas selected from at least one of a hydrogen and an inert gasso that a difference between top and bottom points of the roughness ofthe surface of the crystalline semiconductor film is 6 nm or less. 23.The method for manufacturing the semiconductor device according to claim22, wherein the inert gas is selected from the group consisting ofnitrogen, argon, helium, neon, krypton and xenon.
 24. The method formanufacturing the semiconductor device according to claim 22, whereinthe insulating layer is a single layer film structure or a stackedstructure of two insulating films formed on a glass substrate.
 25. Themethod for manufacturing the semiconductor device according to claim 22,wherein the laser beam is a linear laser beam.
 26. The method formanufacturing the semiconductor device according to claim 22, whereinthe crystalline semiconductor is heated at 450° to 600° after levelingthe surface of the crystalline semiconductor.
 27. The method formanufacturing the semiconductor device according to claim 22, furthercomprising a step of hydrogenating the crystalline semiconductor film.28. A method for manufacturing a semiconductor device comprising:forming a crystalline semiconductor film over an insulating surface;irradiating the crystalline semiconductor film with a linear laser beamin a gas selected from at least one of a hydrogen and an inert gas tolevel a surface of the crystalline semiconductor film, wherein thecrystalline semiconductor film is hydrogenated by a heating process. 29.The method for manufacturing the semiconductor device according to claim28, wherein the inert gas is selected from the group consisting ofnitrogen, argon, helium, neon, krypton and xenon.
 30. The method formanufacturing the semiconductor device according to claim 28, whereinthe insulating layer is a single layer film structure or a stackedstructure of two insulating films formed on a glass substrate.
 31. Themethod for manufacturing the semiconductor device according to claim 28,wherein the crystalline semiconductor is heated at 450° to 600° afterleveling the surface of the crystalline semiconductor.
 32. A method formanufacturing the semiconductor device comprising: disposing a substrateon a stage where a semiconductor film is formed over the substrate;floating the substrate over the stage by supplying a gas to a side ofthe substrate which faces toward the stage; irradiating thesemiconductor film with a linear laser beam while the substrate isfloated; and moving the substrate while irradiating the semiconductorfilm with the linear laser beam.
 33. A method for manufacturing asemiconductor device according to claims 32, wherein the gas is jettedfrom holes in the stage.
 34. A method for manufacturing a semiconductordevice according to claims 32, further comprising a step of eliminatingdust from a surface of the semiconductor film by jetting a gas from anozzle.
 35. A method for manufacturing a semiconductor device accordingto claims 32, wherein the semiconductor film is crystallized by thelaser beam.
 36. A method for manufacturing a semiconductor deviceaccording to claims 32, wherein a surface of the semiconductor film isleveled by the laser beam.
 37. A method for manufacturing asemiconductor device according to claims 32, wherein an oxidized filmformed on the semiconductor film is abraded by the laser beam.
 38. Asemiconductor device comprising: an insulating film formed on a glasssubstrate; a crystalline semiconductor film formed on the insulatingfilm; a gate insulating film formed on the crystalline semiconductorfilm; and a conductive film formed on the gate insulating film, whereina surface of the crystalline semiconductor film has a roughness, whereinthe roughness has a difference between top and bottom points of 6 nm orless.
 39. A semiconductor device according to claim 38, wherein an oxidefilm is formed on the crystalline semiconductor film.
 40. Asemiconductor device according to claim 38, wherein the crystallinesemiconductor film is hydrogenated.
 41. A semiconductor device accordingto claim 38, wherein the glass substrate is 1200×1600 mm or 2000×2500 mmin length.
 42. A semiconductor device according to claim 38, wherein thesemiconductor device is a display device.
 43. A semiconductor deviceaccording to claim 38, wherein the semiconductor device is an activematrix type liquid crystal display device.
 44. A semiconductor deviceaccording to claim 38, wherein the semiconductor device is an electronicapparatus selected from the group of a video camera, a digital camera, arear type or front type projector, a head mount display, a personalcomputer, a portable information terminal, a mobile computer, a mobiletelephone and an electronic book.
 45. A method for manufacturing asemiconductor device comprising: forming a crystalline semiconductorfilm over an insulating surface, wherein a surface of the crystallinesemiconductor film has a roughness; leveling a surface of thecrystalline semiconductor film by irradiating the crystallinesemiconductor film with a laser beam in a gas selected from at least oneof a hydrogen and an inert gas so that a difference between top andbottom points of the roughness of the surface of the crystallinesemiconductor film is 3 nm or less.